Radar systems traditionally transmit electromagnetic pulses at a nominal center frequency toward the region in which targets are expected to be found, and detect the presence of the target by receiving reflected pulses during the inter-transmit-pulse interval. It has long been known that these transmitted pulses have an overall bandwidth which is a function of the pulse width, and which can be affected by the rise and fall times of the pulses, as well as the width and shape of the pulse envelope (including amplitude modulation and the shape of the leading and trailing edges), and the pulse's phase (including frequency) modulation. The bandwidth of high-power electromagnetic pulses with rapid rise and fall times is such that significant energy components occur in frequency ranges occupying frequency sub-bands bands away from the nominal center frequency or bandwidth of the radar. In this context, a radar band is considered to be a named radar band such as C-band or X-band, which contains sub-bands, as for example the 3 MHz sub-band 9800 MHz to 9803 MHz within the X band. These energy components may interfere with such equipments occupying other frequency sub-bands away from the bandwidth of the radar. In times past, the interference was usually manifested in non-radar equipment in these other frequency ranges or sub-bands, because of the relatively high power of the radar pulses, although some interference with the radar receiver by other equipments occasionally occurred.
In the sixty or so years since the introduction of radar, many sophisticated schemes have been used to improve the efficacy of radar. One such scheme allows a radar system to detect targets, at the same antenna beam position, at both short and long distances. This is accomplished by the use of constant-amplitude transmitted pulse waveforms divided into two contiguous portions. A first of the two contiguous portions of the transmitted pulse waveform includes a high-energy long-duration subpulse for long-range detection, and the second portion includes a low-energy short-duration subpulse for short-range detection.
Also in the years since radar was introduced, the number of other users of the electromagnetic spectrum has grown, and many of these additional users operate in the same general frequency bands as radar systems. The broad bandwidth and high power of radar pulses still causes interference in equipments operating in nearby frequency bands, but there are now many more such equipments than in the past. To avoid overlapping bandwidths, a buffer or isolation band of frequencies is normally introduced between the frequency band of a radar and the operating frequency band(s) of other equipments lying within a line of sight extending to the radar horizon. When it is desired to install a new radar system, there may now be few, if any, operating frequencies available for a radar system, if interference with existing installations of other equipments is to be avoided. In extreme cases, a radar system may be limited to use only in wartime. Even if operation of a radar in peacetime is allowed, the paucity of available operating bandwidth and its effect on frequency diversity may compromise system performance.
Reduced radar system interference is desired.